Kamek and Dry Bones are coming further down the line, and you can unlock Pauline through the online tournament next month.
Pauline kind of gets pushed out of the spotlight in the Mario series in favour of Princess Peach, but she's been around ever since she first got kidnapped by Donkey Kong in 1981. Don't worry if you've forgotten about her though, because you probably remember her musical number from Super Mario Odyssey more prominently, and now she's coming to Mario Tennis Aces.
A new trailer reveals she's coming to the game sometime in March, and as reported by Nintendo Life, you can unlock her early by playing the online tournament running for the entirety of the month. Alternatively, you can also wait until she's available for all in April.
The Japanese Nintendo website also reveals Kamek and Dry Bones will be hopping onto the court in April and May as well, which you can see in the screenshots below.